bonding: fix bond_arp_rcv() race of curr_active_slave

bond->curr_active_slave can be changed between its deferences, even to
NULL, and thus we might panic.

We're always holding the rcu (rx_handler->bond_handle_frame()->bond_arp_rcv())
so fix this by rcu_dereferencing() it and using the saved.
Reported-by: Ding Tianhong <dingtianhong@huawei.com>
Fixes: aeea64ac ("bonding: don't trust arp requests unless active slave really works")
CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Acked-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -2254,6 +2254,7 @@ int bond_arp_rcv(const struct sk_buff *skb, struct bonding *bond,
 		 struct slave *slave)
 {
 	struct arphdr *arp = (struct arphdr *)skb->data;
+	struct slave *curr_active_slave;
 	unsigned char *arp_ptr;
 	__be32 sip, tip;
 	int alen, is_arp = skb->protocol == __cpu_to_be16(ETH_P_ARP);
@@ -2299,6 +2300,8 @@ int bond_arp_rcv(const struct sk_buff *skb, struct bonding *bond,
 		 bond->params.arp_validate, slave_do_arp_validate(bond, slave),
 		 &sip, &tip);
 
+	curr_active_slave = rcu_dereference(bond->curr_active_slave);
+
 	/*
 	 * Backup slaves won't see the ARP reply, but do come through
 	 * here for each ARP probe (so we swap the sip/tip to validate
@@ -2312,11 +2315,12 @@ int bond_arp_rcv(const struct sk_buff *skb, struct bonding *bond,
 	 * is done to avoid endless looping when we can't reach the
 	 * arp_ip_target and fool ourselves with our own arp requests.
 	 */
+
 	if (bond_is_active_slave(slave))
 		bond_validate_arp(bond, slave, sip, tip);
-	else if (bond->curr_active_slave &&
-		 time_after(slave_last_rx(bond, bond->curr_active_slave),
-			    bond->curr_active_slave->last_link_up))
+	else if (curr_active_slave &&
+		 time_after(slave_last_rx(bond, curr_active_slave),
+			    curr_active_slave->last_link_up))
 		bond_validate_arp(bond, slave, tip, sip);
 
 out_unlock: